He knew nothing — nothing! — about those controversial lane closures at the George Washington Bridge in September. That’s the official conclusion of their internal investigation, the actual content of which we have yet to see.

What we know already, though, is that it involved 70 interviews, none of which was with any of the three people at the center of the scandal.

Bridget Kelly, who infamously wrote, “Time for some traffic problems in Fort Lee”; Bill Stepien, the governor’s former campaign manager; and David Wildstein, Christie’s appointee at the Port Authority of New York and New Jersey, all refused to be questioned.

And thanks to that diligence, taxpayers are now paying for the same thing three times: Once for a real investigation by the U.S. Attorney’s office, again for the Legislature’s probe on behalf of New Jerseyans, and now for the governor’s overpriced, artificial knockoff.

Imagine what U.S. Attorney Chris Christie would have said. We’re guessing nothing — he would have laughed. Can you imagine him deciding not to indict somebody because an internal probe had found the person innocent?

Surely he’d be skeptical about a lawyer like Randy Mastro, leading an investigation into a governor he’s politically tied to. Mastro’s firm, Gibson Dunn & Crutcher, has long served as Christie’s private counsel. And Mastro himself represented the Port Authority, the agency that created those traffic problems in Fort Lee. That’s a huge conflict of interest.

Regardless, it shouldn’t have cost a penny for Christie to find out what went on in his own administration. He should have asked the people on his payroll, before he fired Kelly and accepted resignations from Wildstein and Bill Baroni, his top Port Authority appointee.

Did Christie really send Baroni to the Legislature to talk about a traffic study, without even having enough interest to ask about it himself?

Mastro’s firm is also investigating Hoboken Mayor Dawn Zimmer’s allegation that Christie’s top officials threatened to withhold Hurricane Sandy aid if she didn’t support his favored project. One million bucks says it finds Christie innocent again.

The governor paid a lot of money to have his lawyers find nothing. Now he owes taxpayers a refund.